A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
During a lithographic process, the substrate is supported on a movable substrate support. With this, a position measurement system is used for determining positions of the support during calibration and/or during the lithographic process itself. A known position measurement system includes an encoder system with four encoders which are positioned at the respective corners of a table of the substrate support. During position measurement a frequent switching needs to take place between the four encoders. This is desirable in order to each time have at least three of the encoders not being blocked by a lens or other part of the apparatus with the table moving there below. The switching requires consistency between the sensors, that is to say no matter which of the sensors are functional at a specific moment in time, together they will always indicate the correct position. The consistency in turn requires the substrate support to be low sensitive to thermal changes. For this it is known to manufacture the substrate support out of a low expansion material, for example a material called Zerodur. In order to at the same time be able to have the support obtain high accelerations during the lithographic process, the support is made as light weight as possible. This for example is done by making the table of the support be relatively thin walled and/or to design it with a ribbed structure with strengthening ribs delimiting thin walled panels there between.
During a lithographic process the substrate support is thermally polluted by heat coming for example from its the long stroke drive motor. Warm air coming from this long stroke drive motor moves upward and heats up the rest of the substrate support. Especially the strengthening ribs and thin walled panels extending between the encoders may expand because of this thermal pollution. The relative thin walled substrate table will react fast on temperature changes in the surrounding. Even though the chuck is made of low expansion material, it still has appeared that this tends to reduce the consistency between the sensors. For example a temperature change of 100 mK and distances between the respective sensors of approximately 0.5 m, with a material like Zerodur leads to 1 nm like position changes, of which approximately 70% goes into inconsistency between the respective sensors. For exposure/readout intervals of the substrate during a lithographic process of 2 minutes or more, this may introduce errors in the position measurement in the order of 0.5 nm-1 nm.
Another known position measurement system includes a substrate support table with gratings and/or interferometer mirrors extending over substantially the entire lengths of its circumferential side walls. When using such a support table equipped with interferometer mirrors and grating plates, the problem of thermal pollution leading to errors in the position measurements of both systems during a lithographic process can be even bigger because the two systems can react differently on thermal pollution, and therefore using the output of both two systems can lead to different calculated support positions. It has been tried to strengthen the side walls with strengthening ribs in order to diminish its thermal expansion behaviour somewhat. However, even then the expansion still occurs to be too large. Furthermore, strengthening of the side edges by for example making the Zerodur table thicker at these side edges, would make the total substrate support too heavy for it to be able to achieve the desired high accelerations.